This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Meropenem, a carbapenem, belongs to an antibiotic class that possesses one of the broadest spectra of antimicrobial activity available, including most of the bacterial pathogens responsible for serious, life-threatening infections occurring in young (<91 days) infants. Meropenem is stable against hydrolysis by most extended spectrum beta-lactamases and AmpC chromosomal beta-lactamases underscoring the drug s activity against many antibiotic resistant Gram positive (e.g., penicillin-resistant S. pneumoniae) and Gram negative (e.g., P. aeruginosa) bacteria. Important indications for meropenem involve infections due to multi-drug resistant pathogens and polymicrobial sepsis. Meropenem is FDA-labeled for pediatric subjects from three months of age through adolescence as single agent antimicrobial therapy for bacterial meningitis and complicated intra-abdominal infections. There is substantial off-label use of meropenem in neonates and infants younger than three months of age. This off-label use occurs despite the lack of adequate meropenem PK, dosing, tolerability and safety data for this vulnerable subject group. The present proposal aims to determine PK and safety of meropenem for the treatment of suspected and complicated intra-abdominal in neonates and infants younger than three months of age. Meropenem will be a safe and tolerable treatment of suspected or documented complicated intra-abdominal infections in infants less than 91 days of age. Multicenter, Prospective, Dose Escalating PK Study of Meropenem (MPODS): This study will evaluate the safety, tolerability and PK-PD of meropenem in infants <91 days of age with suspected and complicated intra-abdominal infections. The specific aims of this trial are: 1. To characterize meropenem single-dose and multiple-dose PK in subjects with suspected and complicated intra-abdominal infections. 2. To characterize the safety profile of meropenem in the treatment of suspected and complicated intra-abdominal infections. 3. To assess collected efficacy data for meropenem for the treatment of suspected and complicated intra-abdominal infections.